1. Field of the Invention
This invention relates to an electrolytic cell useful for the production of sodium hypochlorite from brine.
2. Discussion of the Related Art
It has long been known in the art that the electrolysis of brine produces chlorine, sodium hydroxide, and hydrogen according to the equation:
2NaCl+2H2O= greater than 2NaOH+Cl2+H2
or, alternatively, sodium hypochlorite and hydrogen according to the equation:
NaCl+2H2O= greater than NaOCl+2H2.
The methods and apparatus utilized in performing the electrolysis have varied widely, but one factor has remained constantxe2x80x94the need for large tanks and multiple anodes and cathodes in order to obtain the desired amount of product. Such equipment is bulky, expensive, and inefficient, however, as represented by U.S. Pat. No. 3,721,619 to Ruehlen which discloses a large electrolytic cell holding tank in which multiple cells are utilized. The individual cells have a rod-like anode surrounded by an auxiliary cathode of a steel or stainless steel screen or perforated sheet and further surrounded by an additional cathode. The anode consists of a core of a metal conductor such as copper surrounded by a first layer of nonporous carbon and a second layer of porous carbon.
In addition, there are a variety of other electrolytic cell designs which have been proposed utilizing a coaxially-arranged anode and cathode. Some of these are represented by the following patents.
U.S. Pat. No. 3,076,754 to Evans discloses an electrolytic cell having a hollow anode and hollow cathode arranged coaxially, with the anode surrounding the cathode. The electrolyte passes between the electrodes and water is pumped through the interior of the cathode to cool the cell. The reference uses titanium or platinum-coated titanium for the electrodes of the cell due to the allegedly superior erosion resistance of these materials.
U.S. Pat. No. 3,390,065 to Cooper discloses an electrolytic cell consisting of coaxial tubular electrodes, with the inner electrode preferably being the anode. Cooper also discloses the use of cooling water pumped through the interior electrode. A diaphragm is located between the two electrodes to divide the annular space into an anode compartment and a cathode compartment. This reference discloses using titanium or noble element-coated titanium as the electrode material.
U.S. Pat. No. 3,984,303 to Peters et al. discloses an electrolytic cell having hollow liquid-permeable electrodes arranged coaxially, the anode within the cathode. A tubular ion-permeable membrane is located on the outside of the anode to separate the anolyte layer from the catholyte layer. The cathode is formed from iron, mild steel, nickel, or alloys thereof. The anode is a valve metal such as titanium, tantalum, zirconium, tungsten, or the like, and has a coating of a platinum-group metal or of mixed oxides of valve metal and platinum-group metal. No coolant is pumped through the hollow interior of the anode.
U.S. Pat. No. 4,784,735 to Sorenson discloses an electrolytic cell having an interior tube for recycling catholytic fluid surrounded coaxially by a liquid-permeable cathode, an ion permeable membrane, and then a liquid-permeable anode. Coolant is not pumped through the cell. To obtain the liquid-permeability, the electrodes constitute, for example, perforated or punched plate, or woven wire. The anode metal may be tantalum, tungsten, columbium, zirconium, molybdenum, or alloys containing such metals, but is preferably titanium. The disclosed cathode materials are iron, nickel, lead, molybdenum, cobalt, or alloys containing large amounts of such metals.
Variations of the physical form of the electrodes have also been introduced, as shown by U.S. Pat. No. 4,481,303 to McIntyre et al., which discloses particles suitable for use as electrode material. The particles consist of a substrate such as graphite at least partially, but preferably, fully covered by a coating, the coating containing a binder and a conductive catalyst. The substrate particles may be smaller than about 0.3 mm or larger than about 2.5 cm, but the preferred range is from about 0.7 mm to 4 mm (700-4000 xcexcm).
As can be seen from these patents, those in the art have continued to strive for more efficient, less expensive electrolytic cells. The differing cells reflected by the patents above show the wide variety of approaches that have been taken.
The need exists, however, for electrolytic cells which are able to produce chlorine from brine more efficiently and less expensively.
According to the present invention, sodium hypochlorite with active chlorine ions in aqueous solution is produced from brine using an electrolytic cell having cylindrical metallic electrodes that can include a fixed bed of minute particulate matter to increase the surface area of the electrodes.
The electrolytic cell of the present invention comprises a hollow metallic cylindrical cathode and an metallic cylindrical anode disposed coaxially within the cathode to define an annular passage therebetween. The passage may contain particulate carbon having, for example, an average diameter of about 1000 micrometers.
The electrolytic cell of the invention can also include a cylindrical membrane disposed within the annular passage to divide it into anodic and cathodic chambers.
The method for using an electrolytic cell of the present invention includes passing a solution through the annular passages provided between the hollow cylindrical cathode and a hollow cylindrical anode disposed coaxially within the cathode and applying a voltage to the cell to electrolytically produce a hypochlorite solution. A plurality of electrolytic cells can be employed sequentially in accordance with the invention. Heat exchangers can be positioned in series between the electrolytic cells to control the temperature of solution passing between the cells. The cooling system can include an external heat exchange system, double for divided cell and single for undivided cell.
Both the foregoing general description and the following detailed description contain examples of the invention and do not, by themselves, restrict the scope of the appended claims.